Magnetic disks are generally used for reading and writing data in connection with various equipment such as personal computers and word processors. However, owing to limited disk size, data written on the disk is often compressed, to increase the volume of data stored within a given area on the disk.
To meet this requirement, an apparatus which can read and write data having different periods onto one disk has been developed to increase the volume of data that can be stored on the disk. The apparatus which is disclosed in U.S. Pat. No. 4,805,047 entitled "Read/Write Magnetic Disk Apparatus Operable in Plural Density Modes," is a disk drive capable of reading and writing data while using plural density modes.
Referring to FIG. 1, the data read-out circuit in the disk drive of the above patent includes: a head 10 for reading out data stored on a disk 1; a gain controlled amplifier 11; a first filter 13 for filtering servo data in the data supplied from head 10 via gain-controlled amplifier 11; a second filter 14 for filtering information data in the data supplied from head 10 via gain-controlled amplifier 11; a signal controller 15 for recognizing the frequencies corresponding to the servo data and information data having different bandwidths from each other, and generating a signal for activating one filter corresponding to the recognized frequency; a differentiator 16 for differentiating the signal filtered in first and second filters 13 and 14; a differential comparator 17 for eliminating a distorted signal; and a pulse generator 18 for producing one-shot pulse from the servo and information data. Here, gain controlled amplifier 11 amplifies the servo and information data supplied from head 10, and then supplies the amplified data to first and second filters 13 and 14. At this time, one filter is selected to filter the data by generating a servo signal, so that signal controller 15 activates either first filter 13 when the signal supplied from head 10 corresponds to the servo data or second filter 14 when the signal from head 10 corresponds to the information data.
Meanwhile, the frequency bands employed in the servo and information data may be identical to each other, but the frequency band of the information data is generally higher, so that much more data can be written in this section. Currently, a zone bit recording mode is utilized to maximize the recording density of the information data, wherein the servo data has the same frequency throughout every zone on a disk; but the data on the region where the information data is recorded, is recorded and reproduced as frequencies having different bands for the respective zones.
Therefore, in the conventional disk drive, the greater difference between the frequency band corresponding to the data written in the servo and information data sections induces greatly different signal qualities of the servo and information data. As a result, the function of the filter is lowered which affects the quality of the disk drive.